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For logistical reasons, the military requires that jet fuel (JP-8, F-34) be used in both jet engines and diesel engines. While JP-8-fueled diesel engines appear to operate successfully in many cases, negative impacts, including engine failures, are occasionally reported. As diesel combustion with JP-8 has not been explored in great detail, fundamental information about JP-8 fuel spray combustion is needed. In this study, we report measurements of liquid-phase penetration length, vapor penetration, and ignition delay made in an optically-accessible combustion vessel over a range of high-temperature, high-pressure operating conditions applicable to a diesel engine. Results show that the liquid-phase penetration of JP-8 is less than that of diesel, owing to the lower boiling point temperatures of JP-8. Despite the more rapid vaporization, the vapor penetration rate of JP-8 matches that of diesel and ignition does not advance. In fact, with no required cetane number specification for JP-8, ignition delay times are 25-50% longer for this 38-cetane-number JP-8 fuel sample compared to a 46-cetane-number #2 diesel sample. High-speed shadowgraph imaging shows that a cool flame precedes ignition for both diesel and JP-8 but the time of the cool flame heat release is delayed for JP-8, consistent with the overall ignition delay trend.